Chromosomes, when paired, occur as both autosomes and allosomes. A pair of autosomes are identical in form, size and behavior. This is not necessarily the case for allosomes. Whether the allosomes in an organism are homozygous, heterozygous, diploid or haploid is typically used to determine the sex of that organism.

In the absence of allosomes and environmental factors, what mechanism(s) would allow autosomes alone to determine the sex of an organism?

EDIT: There are two such autosomal mechanisms of which I am aware exist on Earth.

  • Sex chromosomes are speculated to have evolved from chromosomes with a sex-determining gene. Phenotype was determined by the presence of a dominant allele. Later the chromosomes containing the dominant allele inverted and became virtually unable to crossover with their homologue. If this allele resulted in males, it gave rise to the XY system; if females, the ZW system. Some animals still use alleles, so lack true allosomes because these chromosomes may crossover.
  • Fungi do not have sexes because all individuals produce the same type of gamete. Reproductive compatibility is determined by mating types which, as the name implies, work similarly to blood types: if two individuals share the same mating type, they cannot reproduce. This mechanism is not applicable to organisms with sexes.
  • $\begingroup$ No, alternative genetic arrangements are possible, especially in alien life-forms we might encounter. That is the only possible answer that isn't too broad, so I'm going to vote to close. Please ask a more specific question about alternative genetic schemes. $\endgroup$ – kingledion Oct 14 '16 at 15:41
  • $\begingroup$ If we cant Determine Sex via Chromosomes like in Humans, but the system is Similar, than the primary option is that the being is both sexes, and either reproduces asexually, or can change Genders (unless we go Asari from Mass effect, where there is only one gender, and their children are clones with forced mutations gathered from another individual, possibly of other species) Unfortunately, I don't understand this stuff enough to make this an answer, Though perhaps you could expand the question to help describe the viability of these ideas. $\endgroup$ – Ryan Oct 14 '16 at 17:06
  • $\begingroup$ @Ryan: sex could be determined by a gene rather than a whole chromosome. Sex has two phenotypes: male and female. This phenotype is determined by the presence or lack thereof of a dominant allele. Uniquely, having two dominant alleles would be virtually nonexistent because such an individual would only ever have offspring of their own sex. $\endgroup$ – Anonymous Oct 14 '16 at 18:50
  • $\begingroup$ After some more research I found an article that seems to explain why allosomes only coincide with sex-determination. If two autosomes begin to diverge then genetic drift will remove them, but sex-determination ensures that the allosomes are conserved because the species can't survive with only one sex. $\endgroup$ – Anonymous Oct 14 '16 at 19:31

Sex determination systems in Earth creatures are many and varied. Some don't involve chromosomes at all - for instance the temperature determined sex of many reptiles, or fish like wrasse or clown fish which start life as one sex and then transform into another.

Human sex determination involves genes on the sex chromosomes AND on the autosomes. The book Sex Itself: The Search for Male & Female in the Human Genome by Sarah Richardson gives the history of how biologists worked out what's going on.


It could always just be random you know. If it isn’t determined by genetic or environmental factors then the developing organism will simply need to make a coin toss at an early stage in development. Biological systems have tons of random noise and fluctuations occurring constantly so this really isn’t that hard. In fact lots of these sorts of biological coin tosses occur during development. You just need what is called a “bistable switch”. Essentially if you have two positive feedback loops that are mutually inhibitory then only one of those 2 loops can be on at a time.

So say there are two genes M and F. Each encodes a transcription factor that will turn on or off other genes. M’s transcription factor does 3 things. It turns on genes that will produce testosterone and anti-mullerian hormone and eventually male gonads. It also directly or indirectly activates M’s own expression creating a positive feedback loop. And finally it acts to downregulate and decrease the expression of F. F’s transcription factor does the same things, creating a female organism, its own positive feedback loop, and inhibiting M. Now we’ve created a system in which there are two stable states. M on and F off or F on and M off. If both genes are off then as soon as one is activated slightly by random chance it will continue to turn itself on until it is fully active and the other is fully repressed. If both M and F are actively transcribed then as soon as one gets a small random advantage over the other it will inhibit the other and turn itself on further again resulting in one of the two stable states.

With this sex-determination scheme the sex of the organism is determined by random changes in the relative levels of M and F. You can think of it like a ball perfectly balanced on the peak of a hill or at the L1 Lagrange point between a planet and a star. The ball will fall to the left or the right eventually due to random perturbations and then never reverse. A somewhat similar and very well studied example of this sort of bistable switch is the activity of the gene Xist in random X chromosome inactivation and there are many more.


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